curlhttpsrc: add property to set HTTP method
To allow curlhttpsrc to use HTTP methods other than GET (e.g. HEAD), add a "method" property that is a string. If this property is not set, GET is used.
This property is used by gst_uri_downloader_set_method() when uridownloader is requested to perform a HEAD request. This feature of uridownloader is used by dashdemux when the http-head UTCTiming method is used.
This commit ports this functionality from souphttpsrc to curlhttpsrc. See https://bugzilla.gnome.org/show_bug.cgi?id=752413